Trace-Fossil Evidence for Deep-Water Sedimentation in Cretaceous-Arc-Trench Gap, South-Central Alaska: ABSTRACT

Abstract

The Matanuska Formation (Albian-Maestrichtian) in the Talkeetna Mountains, Alaska, contains marine siliciclastic sediments apparently deposited in an east-west-trending trough extending across south-central Alaska. This trough was situated between a magmatic arc on the northeast, indicated by the Gravina-Nutzotin andesitic volcanics, and a deep-sea trench on the south, indicated by the thick Valdez Group metasediments in the Chugach and Kenai Mountains. Along the southern edge of the Talkeetna Mountains, the arc-trench gap sequence represented in the Matanuska Formation contains biogenic and physical sedimentary structures suggestive of deposition on a submarine slope at bathyal (or greater) depths. Turbidite sedimentation is typical, and trace fossils of the Nereites ichnofacies are common. An ideal cycle consists of a complete Bouma sequence. Although complete cycles are common, the turbidites in any given outcrop are rarely uniform with respect to bed thickness, sand/shale ratio, and completeness of the cycle. The sole of each turbidite sand is commonly marked with horizontal trails (e.g., Nereites, ?Paleodictyon, Planolites, Urohelminthoida, and Zoophycos) as well as groove casts and load structures. The occasional presence of wo d suggests proximity to land, but the trace-fossil assemblage and Bouma sequences indicate bathymetric conditions deeper than a shallow shelf. End_of_Article - Last_Page 443------------